1. Field of the Invention
The present invention relates to the field of positively acting differentials for motor vehicles.
2. Prior Art
Land vehicles, such as automobiles, trucks, buses and the like, typically utilize what has become known as an "open differential" for the final drive system. In such a differential, bevel gears are coupled to the inner ends of left and right collinear axles. These bevel gears engage accompanying bevel gears mounted on a pinion pin in a differential case. The differential case, in turn, has a ring gear thereon, with the ring gear and differential case being driven in rotation about the axis of the axles by a pinion gear on the drive shaft. The angular velocity of the ring gear and differential case determines the average angular velocity of the two axles. However, the bevel gearset within the differential case between the two axles allows one axle to turn faster and the other axle to turn slower than the ring gear and differential case at any particular time. This, of course, is highly desirable in normal driving, as it allows the axle coupled to the outer wheel to rotate faster than the axle coupled to the inner wheel when going around a curve or turning a sharp corner.
It also causes the drive system to deliver the same drive torque to each of the two axles to avoid a tendency for the vehicle to pull to one side or the other when power is applied or removed. The amount of torque that can be transmitted through an open differential is limited to that able to be carried by the wheel with the least amount of traction.
There are certain situations, however, where the aforementioned characteristics of an open differential become undesirable. In particular, when one wheel loses traction, the torque which will be delivered to the wheel with traction will be no higher than the torque delivered to the wheel without traction. For instance, with one drive wheel on ice and the other drive wheel on dry concrete, the torque delivered to the drive wheel on dry concrete will be no higher than can be carried by the wheel on ice. A locking differential, however, will effectively couple the two axles together so that they turn in unison, forcing rotation of the drive wheel with greater traction along with the rotation of the wheel with lesser traction. The locking differential, as opposed to the open differential, can transmit as much torque as can be carried by the wheel with the most traction. High performance vehicles, off-road vehicles and the like may similarly take advantage of the characteristics of locking differentials to improve their traction performance.
The present invention is an improvement over a prior art differential described in U.S. Pat. No. 5,901,618, issued May 11, 1999, which is incorporated by reference. That prior art device is shown in the exploded view of FIG. 1. While FIG. 3 is a view of the assembled differential of the present invention, the improvements of the present invention are internal to the assembly of FIG. 3, and accordingly, FIG. 3 is suggestive of the next higher assembly of the parts of FIG. 1. Referring to FIG. 1, the splined inner end of axle 20 engages mating splines in a coupler 150, with a similar coupler 150 at the opposite side of the assembly similarly mating at the inner end of the other axle, not shown. The axle bears against a pinion pin 34 that provides the inward limit for the axial position of the axle. In some embodiments the ends of the axles 20 bear against a thrust slug (not shown) and the slug in turn bears against the pinion pin to establish the desired inward limit. A locking differential reuses some components of the open differential supplied with the vehicle. In particular, the pinion pin 34 that carries the bevel gears of the open differential may be reused although the bevel gears are not used. For this reason, pin 34 is referred to as a pinion pin even though it does not carry gears in a locking differential of the type shown. In the specific version shown, the axles are retained in position by C clips 24 that establish the outward limit for the axial position of the axle, though in other versions, axle retention may be by way of bearings (not shown) adjacent to wheel ends of the axles and other means.
The couplers 150 have a plurality of teeth on the face thereof which may mate with corresponding teeth on the faces of drivers 52, depending upon the axial position of the drivers. The drivers 52, in turn, have saddle-shaped depressions 70 on the opposite faces thereof for loosely surrounding the pinion pin 34 driven by the differential case 154. (See FIG. 3 for the position of the pinion pin in the overall differential assembly.) The drivers 52 each have springs 36 in angled blind holes in the driver, the springs acting on pin 34 to both elastically encourage the drivers to a position having the pin 34 aligned with the center of the saddle-shaped depressions, and to elastically encourage the drivers axially outward away from the pin 34 into engagement with the couplers. Pins 72 on the drivers 52 fit within slots 74 on the opposing face of the opposite driver and function to control the angular displacement of the drivers to each other.
The drivers 52 must be in close axial alignment with the couplers 150 and be free to move axially to allow engagement and disengagement from the adjacent coupler to provide the locking differential action. The outer diameters of the splined ends of the axles 20 typically do not provide a suitable locating surface for the drivers. Spacers 56 establish and retain the drivers 52 in axial alignment with the couplers 150 and provide sliding surfaces for the drivers. As may be seen in FIG. 2, each spacer is located relative to a coupler by a radial shoulder in the face of the coupler. While FIG. 2 shows the spacer aligned by a shoulder against an inside diameter of the spacer, it will be appreciated that the spacer can also be aligned by a shoulder against an outside diameter. The spacers are closely fitted between the pinion pin 34 and the adjacent coupler 150 to maintain the axial position of the spacer. However, there is sufficient clearance to allow the spacers to rotate relative to the couplers.
In the final assembly, the springs 36 encourage the toothed face of the drivers 52 into engagement with the toothed face of couplers 150, and there is sufficient clearance between the saddle-shaped depressions 70 and pin 34 in the final assembly for either driver to move toward the pin 34 sufficiently to allow the teeth of a driver 52 to ride over the teeth of the associated coupler 150.
The operation of the prior art device may be explained as follows. With the teeth of the corresponding driver and coupler pairs engaged, the differential housing may rotate, carrying pin 34 from contact with one side of the saddle to the other, a displacement of (depending on the size of the design) 4 to 7 degrees. This free travel, or backlash, is essential for correct positioning of the differential components during the transition from driving to coasting and vice versa. The drivers are retained with respect to each other by pins 72 and mating slots 74 for a total rotation, one relative to the other, approximately one half of the backlash described previously. When the pin 34 engages the saddle-shaped depressions 70 on either driver, the force of the contact, by design of the saddles, will be angled outward from the plane of the respective driver and will overcome the component of the reaction force acting opposite created by the inclined edges on the mating teeth on the drivers 52 and couplers 150. For example, saddle angles ranging from 30 to 40 degrees are typically used and create outward axial forces that exceed the inward axial forces created by typical 20 to 25 degree inclines of the coupler and driver mating teeth that would otherwise work to separate the driver from the coupler.
Using the foregoing parameters, consider first the vehicle at rest. Assume the two drivers 52 each engage with the respective coupler 150, and for specificity in the starting condition, that the pin 34 is centered in the saddle-shaped depressions 70 in the drivers 52. With the vehicle in gear and engine driving, the pin 34 begins to rotate about the axis of the axle, through the backlash present and compressing against springs 36 to contact the edges of the saddle-shaped depressions 32 in the drivers, and then on further rotation, to force the drivers and couplers, and thus the axles, into rotation. Because the contact angle between the pin 34 and the saddle-shaped depressions 70 exceeds the angle of the edge of the teeth on the couplers and drivers, the force between the pin and the drivers forcing the same into contact against the couplers 150 will exceed the force between the inclined edges of the teeth on the drivers 52 and couplers 150 otherwise tending to force the drivers back toward pin 34, so that the drivers and couplers will remain in positive engagement, regardless of the torque applied to the differential.
If the vehicle now proceeds to drive around a curve, the wheel on the outside of the curve, and thus the coupler 150 associated with that wheel, will tend to rotate faster than the coupler associated with the inside wheel. Assuming power is still being applied, this causes the driver associated with the outside wheel to begin "gaining" with respect to pinion pin 34, the driver rotating forward to a position wherein the saddle-shaped depressions 70 thereon are no longer in contact with pin 34. At this point, pins 72 and mating slots 74 prevent the further relative rotation of the two drivers but allow coaxial translation. Further gaining of the outside wheel continues to rotate the outside coupler at a speed higher than the other differential components. Now, however, the teeth on the driver associated with the outside wheel are free to climb the inclined planes of the teeth on the driver and coupler, with the driver moving toward the pin 34 against the resistance of the associated springs 36 to allow the teeth of the respective driver to slide over the teeth of the respective coupler, repeatedly as required so long as the difference in wheel rotation speeds exist.
If, when in a curve, the vehicle engine is throttled back to coast and the engine is used as a braking or vehicle slowing device, the same basic interaction of parts described above will occur substantially in reverse, now however with the driver and coupler associated with the outer wheel of the curve being engaged, and the driver associated with the inner wheel of the curve climbing over the teeth on the associated coupler as required to allow the inner wheel on the curve to turn slower than the outer wheel. Similarly, in backing around a curve such as backing out of a parking place, the inner wheel will be the drive wheel, as in powering forward, whereas use of the engine to retard the motion of the vehicle when backing will engage the wheel on the outer side of the turn. However in any event, when power is applied while turning to the point that traction is lost by the drive (inside) wheel, pin 34 will catch up to and forcibly engage the appropriate side of the saddle-shaped depression 70 on the outside wheel driver 52, forcing both drivers into engagement with their associated couplers to force rotation of both axles in unison.
The positive acting differential design shown in FIG. 1 overcomes certain problems found in earlier locking differential designs. In particular, when one wheel begins turning faster than the other, such as when turning into a parking space, one driver will be climbing the teeth on the associated coupler and sliding thereover. When the teeth of the driver again align with the spaces between teeth on the coupler, the driver will fall into engagement with the coupler and shortly thereafter climb the sides of the teeth and again disengage. This makes an audible noise, resulting in a "click, click, click" type sound heard from outside the vehicle.
Secondly, a phenomenon called "cycling" can be induced in manual transmission equipped vehicles. Automatic transmissions do not exhibit the condition because the torque converter always maintains a measure of bias load between the engine and drive axle. With manual transmissions, this event occurs when turning at slow speeds with the clutch pedal depressed, such as when turning into a parking space, temporarily decoupling the transmission from the engine and therefore removing any bias load present on the engaged driver and coupler. When the disengaged driver and coupler teeth pass by each other, they briefly reengage, enabling a load to be placed on the differential and axle components. The components between the differential and the wheel then act like an undamped mechanical spring and release the energy by temporarily accelerating the differential, drive shaft and transmission components. The inertia of these components carries the differential pin against the driver saddle, causing the opposite side driver and coupler to lock and continue to process. The continual wind-up and release will build and eventually become sufficient to "rock" the vehicle driveline and require the transmission be put in neutral or the vehicle stopped in order to cease the cycling.
The positive acting differential design shown in FIG. 1 incorporates a synchro ring 158 and related parts to prevent premature engagement of the driver 52 and the coupler 150 to overcome the above described problems. The synchro ring 158 fits within a groove 160 in the face of each coupler 150. As may be seen in FIG. 2, the groove 160 has an undercut therein into which a projecting flange or ridge 162 will snap, retaining each synchro ring 158 at a fixed axial position with respect to the respective coupler 150. The functional diameter of the synchro ring 158 is slightly less I than the groove 160 so that the synchro ring, once deflected within its elastic range and snapped into position into the respective coupler 150, will remain slightly elastically deformed after installation so as to have adequate drag with respect to the respective coupler to rotate with the respective coupler unless forcibly prevented from doing so. While FIG. 2 shows the synchro ring engaging the recess on its inner diameter, it is also possible that the synchro ring could instead engage the recess on its outer diameter.
The spacers 56 each have a paddle-like projection 64 added thereon, which fit into a specific location of the synchro ring 158, such as within the split or open space between the synchro ring ends as shown. The paddle width is smaller than the opening within the synchro ring in such a manner as to allow rotation of the synchro ring 158 relative to the spacer. Within that given freedom, each synchro ring 158 will rotate in unison with the respective coupler 150, though the synchro ring will be restrained by the paddle 64 when contacting the same and will no longer rotate with the respective coupler should the coupler continue to rotate beyond that specified freedom. In that regard, note that when the paddle is positioned between the synchro ring ends, the force on the synchro ring 158 when the opening therein contacts the paddle 64 is a force tending to open the synchro ring, resulting in reduced or increased frictional engagement of the inner or outer diameter of the synchro ring with the groove 160 in the respective coupler. Spacers 56 in the final assembly have no intentional rotational freedom about the axis of the assembly relative to pin 34, but rather in essence rotate in unison with the pin and, thus, with each other.
Certain teeth on the face of drivers 52 extend to a diameter different than the teeth on the couplers, and the remaining teeth on the drivers. A protrusion is formed by these extended teeth that is intended to work with slots in the synchro ring, and is noteworthy to mention that a circular pattern of protrusions could be formed separate from the teeth and be just as functional, however, perhaps not as efficient to produce as the extended teeth. These protrusions have the same spacing as the slots 176 in the synchro rings 158 and will fit within the slots 176 with an angular freedom between a driver 52 and a synchro ring 158 ranging from a minimum of half the saddle backlash to a maximum equal to the angle between driver teeth plus half the saddle backlash.
When the vehicle is powering forward, powering in reverse, using the engine to retard forward motion, or using the engine to retard rearward motion, pin 34 initially rotates the spacer 56 and paddle 64 to contact the synchro ring 158. At the same time, the pin 34 is forced against the saddle-like depression 70 in the driver so that no further clockwise rotation of the paddle with respect to the driver 52 may occur. Under this condition, the pin 34 pushing against the edge of the saddle-like depression in the driver forces the driver into engagement with the teeth of the coupler 150 to provide a positive drive for the respective axle of the vehicle.
When one wheel begins to rotate slightly faster than the opposite wheel, as when the vehicle begins to be powered around a curve, the coupler 150 for the faster wheel drives the driver to a position advanced with respect to pin 34. However, assuming the other axle is still being driven, the driver 52 for the faster wheel will reach the limit of its rotational capability with respect to the opposite driver of 1.5 degrees because of the engagement of the pins 72 and the slots 74. As the coupler continues to rotate faster than the coupler for the wheel still being driven, the teeth on the driver 52 begin to climb the teeth on the coupler 150. As the teeth climb, the engaging portion of the synchro ring 158 approaches the paddle 64 and the extended teeth 52 approach the edge of the slots in the synchro ring 158. After such climbing has been completed, the top of the teeth on the driver 52 are free to slide across the top of the teeth on coupler 150. Further, the synchro ring 158 is free to move to a position slightly under the top of the extended tooth 78.
As the coupler 150 continues to rotate relative to the driver, the synchro ring 158 will continue to rotate with the coupler 150 until it contacts the paddle 64. This will bring synchro ring 158 under the extended teeth on driver 52. Because the spacer 56 is essentially locked to the pin 34 and the driver 52 is limited against further motion by pins 72 and slots 74 between the two drivers, the driver 52 and the spacer 56 and the synchro ring 158 will remain in this position, the synchro ring slipping on the coupler as the coupler continues to rotate in the direction shown. On continued rotation of the coupler in the same direction, each time the coupler teeth come into alignment for potential engagement with the driver teeth, the synchro ring 158 will engage the extended teeth in the driver to prevent such engagement. This eliminates the noise of the repeated engaging and disengaging of the freewheeling driver and coupler experienced in the earlier locking differential designs.
Now assume that the vehicle, still being powered, comes out of the first curve and enters a curve in the opposite direction. In this case because of the change in relative speed, the coupler 150 begins to rotate in the counter-clockwise direction with respect to the driver 52, initially taking synchro ring 158 therewith. In the first part of the rotation, the extended teeth on driver 52 will align with the slots 76 in the synchro ring 158. When, in this condition, the teeth on the coupler and driver align for engagement, the springs 36 force the driver teeth into engagement with the coupler teeth after which time pin 34 will rotate to engage the saddle-like depressions 70 in driver 52 to begin driving that axle, now the more slowly rotating axle.
When returning to driving straight after a turn, the position of the parts relative to each other will depend upon whether, and to what extent, the driver teeth aligned with the coupler teeth for engagement. Ultimately due to variations in terrain and slight course corrections along a straight path, the previously disengaged driver will become reengaged and transfer power from the pin to coupler or vice versa. In any event, because of the general symmetry of the parts, the operation will be as described, whether powering forward or in reverse around a curve, or using the engine for braking forward motion or rearward motion while going around the curve. Also, of course, when the wheel associated with the driver and coupler which are engaged at any given time begins to slip, the opposite driver and coupler, if not already engaged, will immediately engage, thereby providing the desired torque transferring differential action.
While the positive acting differential shown in FIG. 1 works well, the synchro ring 158 has certain shortcomings. It is desirable that there be a certain amount of force required to move the synchro ring to prevent unpredictable operation and to damp out certain oscillations that occur in the operation of the locking differentials. The design of FIG. 1 relies on the residual elastic deformation of the synchro ring bearing radially against the recess to provide a the required drag. It is difficult to produce the amount of drag desired without inducing excessive deformation. Further, it is necessary to have some axial clearance in the engagement of the lips to prevent binding. This leads to an undesirable axial play in the synchro ring.
As noted above, in a design where the paddle 64 engages the synchro ring at the split opening, contact by the paddle tends to loosen or tighten the engagement of the synchro ring to the coupler, making the operation of the positive acting differential less predictable. Also, the split in the synchro ring and assembly to the groove 160 in the coupler 150 with slight elastic deformation, necessary to provide the frictional drag on the synchro ring, means that the tolerances for the projections of the synchro ring are a combination of the manufacturing tolerances for the projections, the engaging surface of the ring, and the recess.
Accordingly, it is desired to provide a positive acting differential with a synchro ring assembly that reliably provides a desired frictional fit and positive axial positioning. Further, it is desired to provide a synchro ring assembly that allows the position of projections on the synchro ring to be held to a desired tolerance.